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UsbDP reaction to reverse polarity?


I am sending some sensors into the woods, with some car-batteries as power-source. They provide 12V most of the time, and has shown to work alright. I am still struggling with getting solar panels and other power-sources to be controlled by the board, but that might be worked on with the tri-power module which is in the works.

Anyway, I am concerned that some people might accidentially switch the red and black wires for the batteries. Can anyone provide some firm answer as to what actually happens when this is done? I have heard about inserting a fuse, any ideas to do this?


Personally, I’d not rely on R/B wire to be enough to protect it. So yes a fuse would be what you’d use to protect from overload, but to protect against reverse polarity you’d need to use a diode as well. Here’s a circuit I’ve found that has a portion of this.

You want to look at the 12v supply portion of the circuit (top right of circuit diagram), there’s a fuse F1, and diode D1 is protecting against reverse polarity (so if the battery positive is connected to the negative terminal, the diode will pass current and essentially becomes a short circuit, and the fuse will then blow). They also have a filter capacitor that is probably not needed in your case since you’ll use the DP to regulate the voltage.

I’d use an in-line fuse holder so you can replace the fuse, something like and have a hard wired connection from the alligator clips (or whatever you have on the battery side of things) through to the 2.1mm barrel connector at the DP module.

If you’re after some input on solar, I have been playing around with some low-price charge controllers, and I’ve had the best luck with this one and at the price, it’s ideal for my tinkering at home. It has a very low power consumption when not charging, it appears to be a true MPPT charger (meaning I get much better recharge in the day than a PWM controller that throws away power), and it’s frankly cheap enough that I could have a few on standby if they die. It’s winter here at the moment, so reduced sun hours, and I am currently running a 12V 6.5AH SLA on a 10w panel, with a Fez Domino powered by the output of the “load” terminals, and the battery has never got to a low voltage situation overnight where it’s turned off. Previously the PWM charge controller would run till about 11pm before shutting down - which I think was a function of high consumption within the controller as well as lower state-of-charge on the battery. I’ve also added a quick voltage divider to allow me to record the battery voltage over time, nothing of concern so far. I personally think this kind of arrangement (don’t try to control the solar charge, just use a commercial controller) is likely to give you the best outcome and least headaches.


you could also use a full wave rectifier and not worry about the polarity from the battery.


The UsbDP has an input diode already so it protected. I agree with Mike that the use of a full wave rectifier and a fuse would be the best.


This is my choice on all new designs in the last year. You never have to worry if they get it wrong. It just works and all for the cost of a suitable bridge rectifier. Cheaper than the support hassles and cost of a replacement board. Happy customer every time.



That’s what I am going for, any specific model you can recommend?


Depends on the total current of your boards. I normally use a 2A device such as this.

Nice and cheap. :slight_smile:


@ Brett - Thanks for your input to the solar-charging stuff, and thanks for the advice on the diode/fuse circuit.